docs: add message format documentation

Document the XADD command structure and example stream entry format.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: psteinroe

src/sink/redis_streams.rs

@@ -4,6 +4,17 @@
 //! 1. `stream` key in event metadata (from subscription's metadata/metadata_extensions)
 //! 2. Fallback to `stream_name` in sink config
 //!
+//! # Message Format
+//!
+//! Each event is added to the stream using `XADD` with:
+//! - Auto-generated stream entry ID (`*`)
+//! - Single field: `payload` containing the JSON-serialized event payload
+//!
+//! Example entry when read with `XREAD`:
+//! ```text
+//! 1704067200000-0 payload {"id": 1, "name": "test"}
+//! ```
+//!
 //! # Dynamic Routing
 //!
 //! The target stream can be configured per-event using metadata_extensions: